The role of gravitational acceleration on human locomotion is not clearly understood. It is hypothesized that the mechanics and energetics of locomotion depend upon the prevailing gravity level. A unique human-rated underwater treadmill and an adjustable ballasting harness were used to stimulate partial gravity environments. This study has two research aspects, biomechanics and energetics. Vertical forces which are exerted by subjects on the treadmill-mounted, split-plate force platform show that peak vertical force and stride frequency significantly decrease (p < 0.05) as the gravity level is reduced, while ground contact time is independent of gravity level. A loping gait is employed over a wide range of speeds (approximately 1.5 m/s to approximately 2.3 m/s) suggesting a change in the mechanics for lunar (1/6 G) and Martian (3/8 G) locomotion. As theory predicts, locomotion energy requirements for partial gravity levels are significantly less than at 1 G (p < 0.05).